1. Field of the Invention
This invention relates to an optical data recording medium, and more particularly to an optical disk for a sampled servo system.
2. Prior Art
An optical data recording medium (hereinafter referred to as an "an optical disk", when applicable) is an optical memory device for recording or reproducing data with a laser beam focused on its recording surface, and therefore the data recording and reproducing operation with the optical disk needs a focusing operation for focusing a laser beam on the recording surface and a tracking operation for causing the focused laser beam to scan the recording surface along tracks.
The focusing operation has been substantially established using means well known in the art. On the other hand, the tracking operation is provided with a so-called "continuous servo system" in which grooves for guiding the focused light beam are employed. Data are recorded in the grooves or in the spaces between the grooves (hereinafter referred to as "on land", when applicable) or reproduced therefrom. A so-called "sampled servo system" in which, with clock signals formed clock pits arranged at predetermined intervals, the signals of servo pits wobbly with respect to the direction of a track, are sampled to perform the tracking operation.
As is apparent from the above-description, the specific feature of the sampled servo system resides in that a tracking operation can be performed without provision of guide grooves in a recording surface.
FIG. 2 is an explanatory diagram showing the arrangement of a conventional optical disk for the sampled servo system. As shown in FIG. 2, tracks 203 are generated on a recording surface as indicated by the broken lines, and clock pits 201 are formed on the tracks 203 of the recording surface at predetermined interval, and in addition servo pits 202 are formed in the recording surface in such a manner that they are wobbly with respect to the direction of a track. The clock pits are arranged so that clock pit signals are generated with a constant time interval at the time of a reproducing operation. As a result, the clock pits are arranged in alignment with a radial direction of a disk with a predetermined angle. Recording pits representing data are not shown in FIG. 2.
In the sampled servo system, the signals of the servo pits 202 are sampled and outputted as tracking error signals. FIG. 3 is a block diagram showing a tracking servo circuit according to the conventional sampled servo system. FIG. 4 is a time chart showing various signals in FIG. 3.
A photodetector (not shown) converts the intensity of light reflected from pits of the optical disk into an electrical signal, which is applied, as a reproducing signal (a), to the tracking servo circuit shown in FIG. 3. In the tracking servo conduit, a peak detecting circuit 10 detects the peak of the reproducing signal, and converts it into a digital pit signal (b), which is applied to a clock pit signal extracting circuit 11. The pit signal (b) includes the signals of servo pit 202, clock pit 201 and recording pit (not shown). The clock pit signal extracting circuit 11 extracts only the clock pit signal (c) from the pit signal (b). The clock pit signal (c) thus extracted is applied to a PLL (phase locked loop) circuit 12, which outputs a clock signal (d), whose frequency is an integer times that of the clock pit signal (c), in synchronization with the latter (c). The clock signal (d) is applied to a sampling timing signal generating circuit 13, which in turn produces sampling timing signals 1 and 2 for sampling changes which are caused in the reproducing signal (a) by the servo pits 202. The sampling timing signals 1 and 2 are applied to a sample and hold circuit I 14 and a sample hold circuit II 15, respectively. In the sample and hold circuits I and II, the signals of two servo pits are sampled with the sampling timing signals 1 and 2, so that sampled reproducing signals (servo pit signals) 1 and 2 are outputted. The sampled reproducing signals 1 and 2 are applied to a differential amplifier 16, which outputs a tracking error signal (e).
The tracking error signal (e) thus provided by the tracking servo circuit is applied to a circuit which, in response to it, operates to displace a laser beam lens actuator to decrease the tracking error, so that a tracking servo is effected to accomplish the tracking operation.
In the conventional optical disk according to the sampled servo system, the clock pits are located only on the tracks as shown in FIG. 2. Therefore, in the case where the tracking operation has been accomplished and the focused light beam is scanning the tracks, the signals of the clock pits can be detected. However, before the tracking operation is accomplished, the focused light beam may scan the inter-tracks. In this case, the signals of the clock pits are decreased, as a result of which the clock signals may not be generated sufficiently. If the clock signals are not generated satisfactorily, then the jitter of the clock signals is increased, and the sampling of the signals of the servo pits is not correctly carried out. As a result, the time required for accomplishment of the tracking operation is increased. In addition, in the case where the tracking operation is not achieved yet, the focused light beam may jump over the tracks at the time of start or access. Therefore, at the time of start, it may be necessary to repeatedly perform the starting operation; and at the time of access, the access time may be increased.